1. Field of the Invention
The present invention relates generally to the field of plant biotechnology and more specifically to enhancing asparagine and protein in plants and seeds.
2. Description of Related Art
Farmers and consumers desire crop plants with improved agronomic traits such as increased yield, increased seed protein production, and improved nutritional composition. Desirable nutritional components of crop plants include, among others, fiber, antioxidants such as Vitamin E, selenium, iron, magnesium, zinc, B vitamins, lignans, phenolic acids, essential amino acids, and phytoestrogens. Although considerable efforts in plant breeding have provided some gains in these desired traits, the ability to introduce specific non-host DNA into a plant genome provides further opportunities for generation of plants with these traits. In particular, while the yield of conventional corn has steadily increased over the years, there has not been a similar increase in the capacity of corn plants to assimilate nitrogen more efficiently or to increase seed protein content.
Availability of nitrogen has a significant positive impact on plant productivity, biomass, and crop yield including the production of seed protein. In plants, inorganic nitrogen is assimilated from the soil, reduced to ammonia, and incorporated into organic nitrogen in the form of the nitrogen-transporting amino acids asparagine, glutamine, aspartic acid and glutamic acid. Asparagine (Asn) is the preferred amide transport molecule because of its high nitrogen to carbon ratio (2N:4C versus 2N:5C) and because it is relatively inert. Asn and other amino acids are also used as building blocks for protein synthesis.
In plants, Asn is synthesized from glutamine, aspartate and ATP, in a reaction catalyzed by the enzyme asparagine synthetase (AsnS). Glutamate, AMP and pyrophosphate are formed as by-products. Two forms of AsnS have been described: a glutamine-dependent form and an ammonia-dependent form. The glutamine-dependent AsnS can catalyze both the glutamine-dependent and ammonia-dependent reactions although glutamine is the preferred nitrogen source.
High concentration of protein is considered an important quality trait for most major crops, including soybean, corn, and wheat. Varieties of high protein corn, wheat, and soybeans, for example, have been identified through traditional breeding. However, most of the high protein lines developed this way have yield drag or other agronomic disadvantages. It would be desirable if the protein content of crops, especially corn, could be increased above the presently available levels, both for human consumption and for use of the product in animal feeds. This would offer the benefit of greatly enhanced nutrient value when the crop is used as food and feed for humans and animals.